The production of zinc oxide by dry and wet processes is known. The classic method of combusting zinc, which is known as a dry process (e.g. Gmelin Volume 32, 8th edition, supplementary volume, p. 772 ff.), produces aggregated particles with a broad size distribution. Although it is in principle possible to produce particle sizes in the submicrometer range by grinding processes, due to the low shear forces which can be achieved, it is not possible to achieve dispersions with average particle sizes in the lower nanometer range from such powders. Particularly finely divided zinc oxide is produced primarily wet-chemically by precipitation processes. The precipitation in aqueous solution generally produces hydroxide- and/or carbonate-containing materials, which have to be reacted thermally to give zinc oxide. The thermal after-treatment has an adverse effect on the finely divided nature since the particles are subjected to sintering processes which lead to the formation of aggregates which are micrometer sized and which can only be broken down incompletely into the primary particles by grinding.
Nanoparticulate metal oxides can be obtained, for example, by the microemulsion process. In this process, a solution of a metal alkoxide is added dropwise to a water-in-oil microemulsion. The hydrolysis of the alkoxides then takes place to give the nanoparticulate metal oxide in the inverse micelles of the microemulsion, the size of which is in the nanometer range. The disadvantages of this process are, in particular, the fact that the metal alkoxides are expensive starting materials, that emulsifiers have to be additionally used and that the production of the emulsions with particle sizes in the nanometer range represents a complex process step.
DE 199 07 704 describes a nanoscale zinc oxide produced by means of a precipitation reaction. In this reaction, the nanoscale zinc oxide is produced starting from a zinc acetate solution by means of alkaline precipitation. The zinc oxide which is centrifuged off can be redispersed to give a sol by adding methylene chloride. The zinc oxide dispersions produced in this way have the disadvantage that, due to a lack of surface modification, they do not have good long-term stability. Likewise due to the lack of surface modification, the production of zinc oxide dispersions is limited to dispersants which are not miscible with water. In addition, the description also mentions the possibility of producing colloidally disperse precipitated zinc oxide, in which a diol-polyol-water mixture using surface modifiers, such as, for example, triethanolamine, is used. In WO 00/50503, which claims the priority of this German application, an example of this is formulated where a mixture of these components is used in a ratio of ethylene glycol:water:triethanolamine in the weight ratio 2:1:0.55. Due to the high content of nonaqueous components, this mixture has significant disadvantages compared with the pure aqueous dispersions. Moreover, the zinc oxide particles here are not equipped with a permanent surface coating.
WO 00/50503 describes zinc oxide gels which comprise nanoscale zinc oxide particles with a particle diameter of ≦15 nm and which are redispersible sols. Here, the precipitations produced by basic hydrolysis of a zinc compound in alcohol or in an alcohol/water mixture are redispersed by adding dichloromethane or chloroform. A disadvantage here is that no stable dispersions are obtained in water or in aqueous dispersants.
In the publication from Chem. Mater “Synthesis and Characterization-of Poly(vinylpyrrolidone)-Modified Zinc Oxide Nanoparticles, Lin Guo and Shihe Yang, 2000, 12”, bauxite zinc oxide nanoparticles are surface-coated with polyvinylpyrrolidone. The disadvantage here is that zinc oxide particles coated with polyvinylpyrrolidone are not dispersible in water.
WO 93/21127 describes a process for the production of surface-modified nanoscale ceramic powders. In this process, a nanoscale ceramic powder is surface-modified by applying a low molecular weight organic compound, for example propionic acid. This process cannot be used for the surface-modification of zinc oxide since the modification reactions are carried out in aqueous solution and zinc oxide dissolves in aqueous organic acids. For this reason, this process cannot be used for the production of zinc oxide dispersions; moreover, zinc oxide is also not specified as a possible starting material for nanoscale ceramic powders in this application.